1. Field of the Invention
This invention pertains to the field of pulsed phase modulated radar systems, and more particularly to an apparatus for detecting and correcting the undesirable frequency non-linearities which arise during the generation and transmission of a pulsed linearly frequency modulated signal.
2. Description of the Prior Art
Pulse compression radar systems are commonly referred to as chirp radar systems. This type of system provides a solution for the conflicting requirements of simultaneous long-range and high-resolution performance in radar systems. These requirements are conflicting because in order to obtain high resolution of a radar target, the transmitted pulse must be very narrow, i.e., wide bandwidth pulses. That is, the higher the desired target resolution, the narrower the transmitted pulse must be. However, long-range target detection requires that the transmitted pulse have a larger power content. Power content is directly proportional to the area under the transmitted pulse. In order for a transmitted pulse to have both high target resolution and long range detection capabilities it will have to be both narrow and envelop a large area. This large area can come only from increasing the height of the pulse, if high target resolution is to be maintained. The peak power required to transmit such a high, narrow pulse is so large as to be impractical.
The chirp technique eliminates the need for a high, narrow pulse and the associated high peak power transmitter. The technique employs a long high-duty factor transmitted pulse, which high duty-factor provides a large power content because of the large area it envelops. The large power content provides for long-range target detection. In addition, the pulse is linearly frequency modulated (chirped) which allows it to cover a frequency interval many times the inherent bandwidth of the envelope. This increase in bandwidth provides for additional target resolution. The chirp technique, then, provides a radar system with both long-range target detection and high target resolution without the need for high peak power transmitters.
In the use of chirp radar systems, however, it is very important that the frequency modulation be extremely linear. However, during the generation and transmission of the chirp pulse, undesirable incidental phase and amplitude distortion is often introduced onto the radar signals in the form of phase and amplitude modulation. This phase and amplitude distortion tends to degrade the chirp radar performance.
Various prior attempts have been made to improve linearity. But, these schemes exhibit high frequency non-linearities. For example, the conventional delay line feedback technique is widely used for generating chirp pulses and yields excellent results at low chirp bandwidths. This technique involves passing the chirp waveform through a delay line whose group delay is constant over the chirp bandwidth and is short compared to the chirp duration. The delayed waveform is mixed with the undelayed waveform, generating a pulse of an intermediate frequency. However, if the delay line is not sufficient length, the intermediate frequency is too low to accept the information rates necessary for applications requiring a wideband control loop, which wideband control loop is necessary for high target resolution in a radar system. Constructing a delay line of sufficient length to provide a high intermediate frequency invariably results in phase non-linearities in the delayed signal. What is desired, then, is a means of linearizing a chirp radar pulse to provide a high power, high resolution radar system using a delay line feedback technique without introducing the phase distortion involved in using a very long delay line.